Image: A Close-Up Look of Two of the Extremely Red Galaxies Revealed By Spitzer

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A close-up look of two of the extremely red galaxies revealed by Spitzer. While they are very faint (the bottom one) or even completely invisible (the top one) in the deepest-ever optical images obtained by Hubble, Spitzer easily picked them up because of their strong infrared emissions. Astronomers believe these galaxies are so red because they are very old, possibly as old as the Universe was  about 2 billion years  at the time when these galaxies emitted the lights that Spitzer received today.